Hydrate Phase Equilibria for Methyldiethanolamine and Empirical

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Hydrate Phase Equilibria for Methyldiethanolamine and Empirical Modeling for Prediction Khalid Alef,*,† Stefan Iglauer,‡ Rolf Gubner,† and Ahmed Barifcani† †

WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Bentley, Western Australia 6102, Australia School of Engineering, Petroleum Engineering Discipline, Edith Cowan University, Joondalup, Western Australia 6027, Australia

‡

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/18/18. For personal use only.

S Supporting Information *

ABSTRACT: The issue of gas hydrates in gas pipelines is commonly addressed by injecting hydrate inhibitors at the well heads. Alongside these inhibitors, other chemical additives are also injected to address various concerns such as to reduce the risk of corrosion and scaling. However, it is not clear how the combined chemical cocktail affects gas hydrate formation over a wide pressure range. Monoethylene glycol (MEG) and methyldiethanolamine (MDEA) are common chemicals that are usually used as part of hydrate inhibition and corrosion control programs respectively. Thus, in this study, the methane hydrate inhibition performance of MDEA in the presence and absence of MEG was assessed. The study produced new hydrate phase equilibria data at a high pressure range, suggesting MDEA performs as a thermodynamic hydrate inhibitor and thus enhances the hydrate inhibitory performance of MEG. Furthermore, because there does not appear to be any flow assurance prediction software that has the capability to simulate the effect of MDEA on hydrate formation, an algorithm that can accurately predict the equilibrium temperature of aqueous MDEA solutions with and without MEG was developed. The algorithm is based on the empirical modeling of the experimental data obtained in this study. This work will thus aid in the industrial application of hydrate inhibitors and improve gas hydrate prevention in production pipelines.

1.0. INTRODUCTION The formation of ice-like solids known as gas hydrates is an ongoing issue in the production of valuable natural resources.1−3 Hydrates are crystalline solids which are composed of gas and water molecules; the gas molecules are known as “guest molecules” which become confined in cagelike cavities formed by water molecules.2,4,5 High-pressure and low-temperature conditions, which are typically experienced within subsea production pipelines, can accelerate the rate of hydrate formation.6 Conventional hydrate inhibition techniques such as thermal insulation, depressurization, and natural gas dehydration by glycol or molecular sieves may be impractical and not cost-effective.7 Thus, chemical hydrate inhibitors are commonly utilized in the industry for hydrate inhibition and prevention of methane hydrate reformation.8−10 They are classified as low-dosage hydrate inhibitors (LDHIs) and thermodynamic hydrate inhibitors (THIs).11 THIs work by moving the hydrate phase boundary toward lower temperatures and higher pressures, thus increasing the hydrate-safe region.12 Monoethylene glycol (MEG) is one of the most commonly used thermodynamic hydrate inhibitors and is utilized in this study.13,14 Alongside hydrate formation, corrosion is another major issue leading to serious cost repercussions and downtime.15−17 A corrosion control program may thus be adopted, which © XXXX American Chemical Society

usually consists of using an amine such as methyldiethanolamine (MDEA) to increase the pH of the fluid system to initiate the precipitation of a stable iron carbonate layer on the inside of the pipeline for surface protection (pH stabilization)18 or by injecting corrosion inhibitors.19,20 The combined use of both gas hydrate and corrosion inhibitors is popular, and some compatibility studies have been conducted in terms of corrosion; however, the impact of different chemical additives on gas hydrate formation needs more work.6,19,21 In this context, Obanijesu et al. studied the effect of different chemical additives such as corrosion inhibitors on the hydrate formation temperature and found that corrosion inhibitors promote gas hydrate formation.6 This may be detrimental as it can increase the risk of hydrate formation and production downtime. Others evaluated hydrate inhibitors as being able to perform as corrosion inhibitors, and in recent studies, hydrate and corrosion inhibitors were combined to form of a single polymer that can tackle both hydrate formation and corrosion issues simultaneously.22,23 However, the combined injection of MDEA and MEG has not been fully explored at a high pressure range, which is, however, relevant for production.24 Chemical additives such as MDEA or corrosion inhibitors may Received: May 29, 2018 Accepted: August 1, 2018

A

DOI: 10.1021/acs.jced.8b00440 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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lead to overinhibition or even under-inhibition of gas hydrates in pipelines. Additionally, chemical compositions of inhibitors and additives are increasingly becoming proprietary due to the commercial appeal and preservation of a competitive edge.25 This leads to an increasing lack of fundamental understanding and increasingly complex prediction models, or the lack thereof, of such prediction tools. Thus, in this investigation, the methane hydrate inhibition performance of one such chemical additive, MDEA, which is commonly injected in combination with MEG, was assessed at a high pressure range (7−20 MPa) that has not been previously tested. Furthermore, an algorithm consisting of empirical models based on the experimental data of this study is provided due to the lack of software predictions for the hydrate inhibition performance of MDEA solutions. The models are based on a linear interpolation scheme between the hydrate phase boundaries of various concentrations of MDEA solutions to accurately predict the equilibrium temperature shift due to the presence of MDEA.

2.0. METHODOLOGY 2.1. Materials and Apparatus. The materials utilized in this study are given in Table 1. MEG was obtained from

Figure 1. Schematic of the high-pressure PVT sapphire cell used in this study which is capable of performing hydrate inhibition testing.

equilibria.2 This method requires that the volume is kept constant while applying a step-cooling process (in this study: 2 °C/h) until hydrate blockage had occurred, and then applying a careful step-heating process (1 °C/h) to dissociate the gas hydrate. The intercept of the pressure−temperature curves from the cooling and heating processes gives the dissociation temperature also known as the thermodynamic equilibrium.2,28 Each test consisted of 4 experiments at varying pressures between 7 and 20 MPa to determine the full hydrate phase boundary. Table 2 contains the experimental test matrix for the

Table 1. List of Materials Used in the Experiments material MEG MDEA deionized water methane nitrogen

formula C2H6O2 CH3N(C2H4OH)2 H2O CH4 N2

purity 99.477% ≥99% 18.18 MΩ·cm (24 °C) 99.995% 99.9959%

source Chem-Supply Sigma-Aldrich Hydro-Check 414R BOC AtlasCorpo, NGP10+

Table 2. Experimental Matrix of Hydrate Inhibition Tests Conducted Using the Isochoric Method

Chem-Supply with a purity of 99.477% (molar). MDEA was sourced from Sigma-Aldrich with a purity of ≥99% (molar). Deionized (DI) water was conveniently produced within the laboratory using Hydro-Check 414R with an electrical resistivity of 18.18 MΩ·cm (24 °C). The hydrate-forming gas was ultrahigh purity methane supplied by BOC with a purity of 99.995% (molar). Nitrogen for purging purposes was generated using an in-house nitrogen generator (AtlasCorpo, NGP10+) with a purity of 99.9959% (molar). A high-pressure PVT cell located in the Clean Gas Technology Australia (CGTA) laboratory, Curtin University, was utilized for the hydrate testing (Figure 1). The cell chamber (60 cm3) and tubing have a total volume of 86 cm3. Before each test run, the cell was cleaned with ethanol and thoroughly rinsed with deionized water. A vacuum pump was then utilized for drying the cell and to remove any remaining contaminants, and finally purged with nitrogen. The cell was equipped with a magnetic stirrer (up to 500 rpm stir rate) to promote mixing between the phases to facilitate hydrate formation and the prevention of a hydrate film to simply form at the gas−liquid surface.26,27 Pressure sensors for measuring pressure and K-type thermocouples were installed for measuring the air bath, vapor, and liquid temperatures within the sapphire cell. The cell was mounted firmly within an air bath operated by a cooling/heating system. The inside of the cell was clearly visible from the outside and aided by a camera system with a light source to enhance the imagery. 2.2. Method. The well-known isochoric method was employed to accurately determine the hydrate phase

composition (wt %) formulation

water

MDEA

MEG

pure water MDEA MDEA MDEA pure MEG MEG−MDEA MEG−MDEA

100 97.5 95 92.5 80 77.5 72.5

0 2.5 5 7.5 0 2.5 7.5

0 0 0 0 20 20 20

isochoric hydrate tests conducted in this study. The test solutions were accurately prepared by precise mass measurement of the various components required using a highly accurate self-calibrated electronic balance with an accuracy of 0.09%. The test solutions were mixed in a beaker while being sparged with nitrogen and magnetically stirred for complete synthesis. An 8 mL sample was then injected into the PVT cell and mixed thoroughly by the magnetic stirrer before the test was initiated.

3.0. RESULTS Initial hydrate testing was conducted for the two reference systems, pure water, and water-MEG (20 wt %) samples to ascertain the accuracy of the results by comparing to the available literature and software data. Figure 2 illustrates the comparison, while the hydrate phase equilibria data are provided in Table 3 and Table 4. The tests were conducted B



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AARE (T ) =

100 n

n

∑ i=1

Tcalc − Texp Texp

(1)

3.1. Pure MDEA Tests. Samples of water−MDEA mixtures at MDEA concentrations of 2.5, 5, and 7.5 wt % were tested for methane hydrate inhibition. It was observed that the level of foaming increased as MDEA concentration increased. The newly obtained equilibria data are provided in Table 3. The hydrate phase boundaries for the MDEA samples are plotted in Figure 3. The hydrate phase boundaries as compared to that of

Figure 2. Methane hydrate phase boundaries for pure water and pure MEG (20 wt %) as compared to literature and software predictions.

Table 3. Equilibria Data for Pure Water and Pure MDEA Samples Measured in This Studya MDEA (2.5 wt %)

pure water

MDEA (7.5 wt %)

MDEA (5 wt %)

P (MPa)

T (°C)

P (MPa)

T (°C)

P (MPa)

T (°C)

P (MPa)

T (°C)

7.11 9.98 15.20 19.83

9.83 13.00 16.61 18.84

7.36 10.07 15.29 19.93

9.85 12.76 16.40 18.57

7.42 9.96 14.98 20.26

9.63 12.28 16.03 18.49

7.66 10.84 15.24 20.02

9.64 12.85 15.89 18.18

Figure 3. Methane hydrate phase boundaries for pure MDEA (2.5− 7.5 wt %) and their equivalent MEG concentrations using Multiflash.

a

Standard uncertainties in pressure and temperature measurements are ±0.05 MPa and ±0.03 °C, respectively.

pure water have shown an average leftward shift by 0.29, 0.58, and 0.82 °C, respectively. This leftward shift confirms that MDEA can act as a thermodynamic hydrate inhibitor even at higher pressures. The tests were simulated in Multiflash, and the results showed no change in the hydrate phase boundary as compared to the hydrate phase boundary of pure water, confirming that the effect of MDEA on the phase boundary has not been taken into account.24 The equivalent MEG concentrations required to yield the same amount of temperature suppression or shift in hydrate phase boundary caused by MDEA was determined by simulation in Multiflash (Figure 3). The results reveal that 2.5, 5, and 7.5 wt % of pure MDEA solutions are equivalent to 1.1, 2.1, and 3.3 wt % of pure MEG, respectively. This suggests that MEG is 2.31 times more effective than MDEA. 3.2. MEG Tests. MDEA was tested in a 20 wt % MEG solution to determine the combined hydrate inhibition performance at high pressures (7−20 MPa). Interestingly, during the cooling and hydrate nucleation phase, bubbling was observed instead of foaming (Figure 4). However, the foaming characteristic of MDEA samples was visible when the solution was stationary after stirring, as seen in the sample bottles in Figure 4. The hydrate phase boundaries are plotted in Figure 5, while equilibria data are provided in Table 4. The hydrate profiles for 20 wt % MEG with added MDEA at 2.5 and 7.5 wt % relative to deionized water show an enhanced hydrate inhibition performance as opposed to a 20 wt % pure MEG solution (Figure 5). At 2.5 wt % of MDEA concentration in the MEG solution, an average hydrate equilibrium temperature suppression of 0.13 °C was produced. While at a concentration of 7.5 wt % of MDEA, an average suppression of 0.46 °C was found. Interestingly, both samples showed a greater shift at higher pressures as opposed to lower pressures. The results clearly

Table 4. New Methane Hydrate Phase Equilibria Data for 20 wt % MEG/Water−MDEA Mixturea pure MEG (20 wt %)

MEG (20 wt %)− MDEA (2.5 wt %)

MEG (20 wt %)− MDEA (7.5 wt %)

P (MPa)

T (°C)

P (MPa)

T (°C)

P (MPa)

T (°C)

7.13 10.24 15.06 19.97

4.13 7.25 10.46 12.72

7.58 10.45 15.42 20.51

4.56 7.23 10.51 12.82

7.67 9.83 15.33 20.08

4.32 6.45 10.08 12.3

a

Standard uncertainties in pressure and temperature measurements are ±0.05 MPa and ±0.03 °C, respectively.

three times under the same conditions for repeatability. The results had a standard deviation of 0.16 °C, indicating a very small deviation in the obtained data. Furthermore, the measured data were compared to software predictions using Soave−Redlich−Kwong (SRK), Peng−Robinson (PR), and CPA fluid packages in PVTSim, Multiflash, and CSMHYD.29−31 The measured data were also compared to similar literature data at the tested pressure range.32−36 An absolute average relative error (AARE) of 1.77% between the measured equilibria data of the different mixtures was found, confirming that our results match very well to published data and predictions. The absolute average relative error was calculated using eq 1, where Texp is the experimentally measured equilibrium temperature and Tcalc is the equilibrium temperature predicted using software or obtained from literature. C



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performance is observed. MDEA and water merge by strong hydrogen bonding, thus making the water molecules less accessible to gaseous guest molecules, resulting in hydrate inhibition.37,38 On the other hand, where carbon dioxide and acids are involved, MDEA has an exothermal reaction which generates heat, promoting dissociation of the gas hydrate.39,40 Furthermore, predictions for combined MDEA and MEG solutions using Multiflash showed an almost negligible temperature suppression as compared to our experimental data (Figure 5). The temperature shift that can be seen in the Multiflash predictions is simply the hydrate phase boundary of the same solution while ignoring the MDEA concentration, thus resulting in a higher MEG proportion. Therefore, the prediction is misleading as it produces results for a MEG solution of 21.6 wt % as opposed to the 20 wt % solution as defined by the user in the case of MEG−MDEA (7.5 wt %). It was assumed that the selected Multiflash configuration and equation of state (CPA) was not capable of recognizing MDEA’s inhibition effect, so different equations of states (i.e., PR, SRK, modified PR, and modified SRK) were selected, but the results remained unhindered, suggesting that the added hydrate inhibitory performance of MDEA has not been taken into account in the Multiflash simulation model. The combined effect of MEG (20 wt %) with MDEA at 2.5 and 7.5 wt % on the hydrate phase boundary was found to be equivalent to the hydrate performance of 20.28 and 20.95 wt % of pure MEG, respectively. The equivalent MEG concentration for the same MDEA concentration is higher for pure MDEA as opposed to combined mixtures of MEG−MDEA. This suggests that a mixture of MEG−MDEA showed a lesser performance as a hydrate inhibitor as compared to pure MDEA by a factor of 3.6. 3.4. Empirical Modeling. Simulations were conducted using Multiflash, which has an option to input MDEA concentration within the aqueous phase. However, the predicted hydrate phase boundaries of the MDEA solutions were identical to the results of pure water (100 wt %). This exposes the software’s incapability to take into account the inhibitory performance of MDEA. One of the goals of this study is to present an algorithm based on empirical modeling to allow for the prediction of equilibrium conditions of aqueous MDEA solutions with MEG. This is very useful since flow assurance software are not able to predict the hydrate inhibitory effect of MDEA. Thus, after establishing the hydrate inhibition performance of MDEA and MEG−MDEA solutions, the next step was to develop a relation between the experimentally measured equilibria data and MDEA concentration. This can be achieved through linear interpolation with the assumption that at a given pressure, the relationship between the thermodynamic equilibrium temperature and MDEA concentration is a linear one. Furthermore, Figure 3 and Figure 5 show that the equilibrium temperature decreases consistently with increasing MDEA concentration. Thus, a simple interpolation scheme that can determine the hydrate equilibrium conditions of MDEA and MEG−MDEA mixtures at different MDEA concentrations (x) based on the experimental data from this study is put forth. Given that MDEA’s hydrate inhibition performance increases with increasing concentration, this increase will result in lower equilibrium temperature as compared to those of pure water (x = 0) or pure MEG (x = 0). This is expressed by eq 2, where the first RHS term (T0), which could also be called the reference term, denotes the equilibrium temperature

Figure 4. Stages of hydrate testing in MEG−MDEA (2.5 and 7.5 wt %) mixtures.

Figure 5. Measured and simulated phase boundaries for combined 20 wt % MEG with MDEA (2.5 and 7.5 wt %).

show that as the concentration of MDEA increases, there is a leftward shift in the hydrate profile, confirming that MDEA is contributing as a thermodynamic hydrate inhibitor even at higher pressures. The high solubility of MDEA in water is a contributing factor to why the enhanced hydrate inhibitory D



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of either deionized water or the pure MEG solution. The second RHS term, ΔTx in eq 2, is simply the temperature shift from T0 to the equilibrium temperature (Tx) of a mixture of x wt % of MDEA. Tx = T0 − ΔTx

By substituting eqs 3 and 5 into eq 2, the general expression shown in eq 6 for calculating equilibrium temperature, T, at a concentration of x wt % of MDEA in pure water or MEG mixtures can be developed. The constants a and b were derived from the exponential expressions of the reference systems, while c and d for both pure MDEA and MEG solutions were obtained from the exponential expressions of the pressure versus equilibrium temperature shifts between the reference systems and high MDEA concentration solutions. These constants are given in Table 5.

(2)

The reference term (T0) for water is calculated by using a fitted exponential trendline on the experimental equilibrium data. The exponential function for the experimental data after correlation is given with pressure (P) as the subject. The equation can be rearranged in terms of T0 as shown in eq 3 (where a and b are constants of the exponential expression). The reference term for the pure MEG solution can also be predicted by an equation of state, thus allowing for a wider MEG concentration coverage compared to the 20 wt % MEG concentration adopted within this study. iPy T0 = a lnjjj zzz kb{

iPy iPy T = a lnjjj zzz − c lnjjj zzzx kb{ kd{

(6)

Table 5. Constants Used in Eq 6 for Pure MDEA and MEG−MDEA Mixtures

(3)

MDEA MEG−MDEA

To derive the equilibrium temperature shift, ΔTx involves developing a relationship to address the shift in hydrate phase boundaries of the reference system versus that of a high MDEA concentration mixture (upper boundary) as a function of pressure. In this study, the upper boundary was selected as the pure MDEA solution at a concentration of 7.5 wt %. The equilibrium temperature shift, ΔT7.5, between pure water as the reference, and pure MDEA (7.5 wt %) as the upper bound was determined over a varying pressure range (7−20 MPa) to account for the temperature dependence on pressure (Figure 6).

a

b

c

d

8.769 8.346

2.296 4.323

−0.0183 0.0104

4333.9 0.031

The model was tested by comparing it with experimental data available in literature and data from this study.24 This comparison is shown in Figure 7 and Figure 8; it can be seen

Figure 7. Comparison of predicted to experimental data for MDEA and MEG mixtures from this study.

that the model fits very well. Most of the values predicted by the model are within 0.07 °C of the experimental data and have an average relative difference of 0.57% (Table 6). It can

Figure 6. Pressure versus ΔT7.5, hydrate equilibrium temperature shift for between water and pure MDEA at 7.5 wt %.

The equation for ΔT7.5 can be derived from Figure 6, it is written here as eq 4. However, to determine the equilibrium temperature shift for a mixture of x wt % concentration of MDEA (ΔTx), the ΔT7.5 term can be used to interpolate by multiplying eq 4 by x/7.5. i 1 yz ij P yz zz zzlnjj ΔT7.5 = jjj k −7.288 { k 4333.9 {

i x zyzlnjij P zyz ΔTx = −jjj z z j k 7.5 × 7.288 { k 4333.9 {

(4) Figure 8. Comparison of predicted to experimental data for MDEA solutions from literature.24

(5) E



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the study has presented an algorithm (provided in supporting information) consisting of empirical models based on the experimental data of this study to provide an estimate for the added hydrate inhibitory effect of MDEA. The hydrate phase equilibria data for MDEA illustrate that various chemical additives that are injected alongside hydrate inhibitors can potentially alter the expected hydrate inhibition performance of the adopted hydrate control program. In this case, it increased the hydrate-safe region and perhaps rendered the system into overinhibition.

Table 6. Statistical Comparison of Model and Experimental Dataa mixture this study: pure MDEA

xMDEA (wt %)

P (MPa)

Texp (°C)

Tcalc (°C)b

ΔT (°C)

RD (%)c

2.50

7.36 10.07 15.29 19.93 7.42 9.96 14.98 20.26 7.66 10.84 15.24 20.02 5.70 6.69 7.66 8.67 5.69 6.65 7.63 8.60 7.58 10.45 15.42 20.51 7.67 9.83 15.33 20.08

9.85 12.76 16.40 18.77 9.63 12.28 16.03 18.69 9.64 12.85 15.89 18.18 7.41 8.96 10.21 11.35 6.94 8.42 9.80 10.75 4.56 7.23 10.51 12.82 4.32 6.45 10.08 12.30

9.92 12.69 16.37 18.70 9.70 12.31 15.93 18.60 9.70 12.79 15.82 18.25 7.60 9.01 10.21 11.30 7.08 8.47 9.69 10.76 4.54 7.21 10.45 12.82 4.35 6.41 10.08 12.31

0.07 0.07 0.03 0.07 0.07 0.03 0.10 0.09 0.06 0.06 0.07 0.07 0.19 0.05 0.00 0.05 0.14 0.05 0.11 0.01 0.02 0.02 0.06 0.00 0.03 0.04 0.00 0.01

0.71 0.55 0.18 0.37 0.73 0.24 0.62 0.48 0.62 0.47 0.44 0.39 2.56 0.56 0.00 0.44 2.02 0.59 1.12 0.09 0.44 0.28 0.57 0.00 0.69 0.62 0.00 0.08

5.00

7.50

Akhfash et al. (ref 24): pure MDEA

3.11

7.25

this study: MEG− MDEA

2.50

7.50

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00440. Effect of MDEA on varied MEG mixtures and outline of algorithm (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Khalid Alef: 0000-0003-1751-5636 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to acknowledge the contribution of an Australian Government Research Training Program Scholarship in supporting this research.

■

a

Standard uncertainties in pressure and temperature measurements are ±0.05 MPa and ±0.03 °C, respectively. bModel. cRelative difference, RD (T) = abs(Texp − Tcalc)/Texp × 100.

REFERENCES

(1) Hammerschmidt, E. Formation of gas hydrates in natural gas transmission lines. Ind. Eng. Chem. 1934, 26, 851−855. (2) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Taylor and Francis Group: Boca Raton, FL, 2008. (3) Carroll, J. Natural Gas Hydrates: A Guide for Engineers; Gulf Professional Publishing: 2014. (4) Koh, C.; Westacott, R.; Zhang, W.; Hirachand, K.; Creek, J.; Soper, A. Mechanisms of gas hydrate formation and inhibition. Fluid Phase Equilib. 2002, 194, 143−151. (5) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D. Thermodynamic model for predicting phase equilibria of simple clathrate hydrates of refrigerants. Chem. Eng. Sci. 2011, 66, 5439−5445. (6) Obanijesu, E.; Gubner, R.; Barifcani, A.; Pareek, V.; Tade, M. The influence of corrosion inhibitors on hydrate formation temperature along the subsea natural gas pipelines. J. Pet. Sci. Eng. 2014, 120, 239−252. (7) McIntyre, G.; Hlavinka, M.; Hernandez, V.; Bryan, T. Hydrate Inhibition with MethanolA Review and New Concerns over Experimental Data Presentation; 83rd Annual GPA Convention: New Orleans, 2004. (8) Kim, J.; Kim, H.; Sohn, Y. H.; Chang, D.; Seo, Y.; Kang, S.-P. Prevention of methane hydrate re-formation in transport pipeline using thermodynamic and kinetic hydrate inhibitors. J. Pet. Sci. Eng. 2017, 154, 114−125. (9) Seo, Y.; Kang, S.-P. Inhibition of methane hydrate re-formation in offshore pipelines with a kinetic hydrate inhibitor. J. Pet. Sci. Eng. 2012, 88, 61−66. (10) Jamaluddin, A.; Kabir, C. Flow assurance: Managing flow dynamics and production chemistry. J. Pet. Sci. Eng. 2012, 100, 106− 116. (11) Kelland, M. A. History of the development of low dosage hydrate inhibitors. Energy Fuels 2006, 20, 825−847.

thus be established that the developed model accurately represents the effect of MDEA on the hydrate phase boundary for MDEA concentrations of 0−7.5 wt % and a pressure range of 7−20 MPa.

4.0. CONCLUSION The combined use of MDEA and MEG is very common.13,19,24,37,41−43 As such, the need for understanding how MDEA affects gas hydrate formation and the inhibition performance of MEG at a wide pressure range becomes important for the integrity of the hydrate control program. This study has produced new methane hydrate phase equilibria data for MEG and MDEA mixtures, confirming that MDEA can act as a thermodynamic hydrate inhibitor at high pressures (7−20 MPa), resulting in the suppression of the hydrate phase boundary. Pure MDEA showed an average equilibrium temperature shift of −0.82 °C at a concentration of 7.5 wt %. The combined effect of MDEA (7.5 wt %) with MEG (20 wt %) showed an equivalent hydrate performance of 20.95 wt % MEG. This shows that where MDEA and MEG are applied together for their respective purposes, the system may be slightly overinhibited due to the added hydrate inhibitory performance of MDEA. The study suggests that with the knowledge of the hydrate inhibitory performance of other chemical additives such as MDEA in the MEG injection stream, an added safety margin can be assumed. Furthermore, F



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(34) Verma, V. Gas Hydrates from Liquid Hydrocarbon−Water Systems; University of Michigan: Ann Arbor, 1974. (35) Rock, A. Experimentelle und theoretische untersuchung zur hydratbildung aus gasgemischen in inhibitorhaltigen wassrigen losungen. Ph.D. Dissertation, Universitat Karlsruhe (TH), Karlsruhe, Germany, 2002. (36) Haghighi, H.; Chapoy, A.; Burgess, R.; Tohidi, B. Experimental and thermodynamic modelling of systems containing water and ethylene glycol: Application to flow assurance and gas processing. Fluid Phase Equilib. 2009, 276, 24−30. (37) Davoudi, M.; Heidari, Y.; Safadoost, A.; Samieirad, S. Chemical injection policy for internal corrosion prevention of South Pars seapipeline: A case study. J. Nat. Gas Sci. Eng. 2014, 21, 592−599. (38) Hossainpour, R. Catalysts for Enhanced CO2-CH4 Exchange in Natural Gas Hydrates: An experimental feasibility study of exchange enhancement by use of chemical additives; The University of Bergen, 2013. (39) Park, S.-W.; Lee, J.-W.; Choi, B.-S.; Lee, J.-W. Absorption of carbon dioxide into non-aqueous solutions of N-methyldiethanolamine. Korean J. Chem. Eng. 2006, 23, 806−811. (40) Xiang, Y.; Choi, Y.-S.; Yang, Y.; Nešić, S. Corrosion of carbon steel in MDEA-based CO2 capture plants under regenerator conditions: effects of O2 and heat-stable salts. Corrosion 2015, 71, 30−37. (41) Nyborg, R.; Dugstad, A. Flow Assurance of Wet Gas Pipelines From a Corrosion Viewpoint. In 21st International Conference on Offshore Mechanics and Arctic Engineering; American Society of Mechanical Engineers: Oslo, Norway, 2002. (42) Glenat, P.; Peytavy, J.-L.; Holland-Jones, N.; Grainger, M. South-Pars phases 2 and 3: The kinetic hydrate inhibitor (KHI) experience applied at field start-up. In Abu Dhabi International Conference and Exhibition; Society of Petroleum Engineers: Abu Dhabi, UAE, 2004. (43) Halvorsen, A. M. K.; Andersen, T. R.; Halvorsen, E. N.; Kojen, G. P.; Skar, J. I.; Biørnstad, C.; Fitje, H. The Relationship Between Internal Corrosion Control Method, Scale Control And MEG Handling Of A Multiphase Carbon Steel Pipeline Carrying Wet Gas With CO2 And Cetic Acid. In Corrosion; NACE International: Nashville, TN, 2007.

(12) Li, X.-S.; Wu, H.-J.; Englezos, P. Prediction of gas hydrate formation conditions in the presence of methanol, glycerol, ethylene glycol, and triethylene glycol with the statistical associating fluid theory equation of state. Ind. Eng. Chem. Res. 2006, 45, 2131−2137. (13) Brustad, S.; Løken, K.-P.; Waalmann, J. G. Hydrate Prevention using MEG instead of MeOH: Impact of Experience from Major Norwegian Developments on Technology Selection for Injection and Recovery of MEG; Offshore Technology Conference: Houston, TX, 2005. (14) Sami, N. A.; Das, K.; Sangwai, J. S.; Balasubramanian, N. Phase equilibria of methane and carbon dioxide clathrate hydrates in the presence of (methanol+ MgCl2) and (ethylene glycol+ MgCl2) aqueous solutions. J. Chem. Thermodyn. 2013, 65, 198−203. (15) Alef, K.; Iglauer, S.; Barifcani, A. Effect of Dissolved Oxygen, Sodium Bisulfite, and Oxygen Scavengers on Methane Hydrate Inhibition. J. Chem. Eng. Data 2018, 1821. (16) Olajire, A. A. A review of oilfield scale management technology for oil and gas production. J. Pet. Sci. Eng. 2015, 135, 723−737. (17) Papavinasam, S.; Doiron, A.; Panneerselvam, T.; Revie, R. Effect of hydrocarbons on the internal corrosion of oil and gas pipelines. Corrosion 2007, 63, 704−712. (18) Nyborg, R. Pipeline Corrosion Prevention by pH Stabilization or Corrosion Inhibitors, Rio Pipeline Conference and Exposition; Brazilian Petroleum, Gas and Biofuels Institute-IBP: Rio de Janeiro, Brazil, 2009. (19) Lehmann, M.; Lamm, A.; Nguyen, H.; Bowman, C.; Mok, W.; Salasi, M.; Gubner, R. Corrosion Inhibitor and Oxygen Scavenger for use as MEG Additives in the Inhibition of Wet Gas Pipelines; Offshore Technology Conference-Asia: Kuala Lumpur, Malaysia, 2014. (20) Lehmann, M. N.; Bowman, C. W.; Mok, W. Y.; Barr, N. J. Application of oxygen scavengers to glycol systems. U.S. Patent US9249516B2, 2016. (21) Luna-Ortiz, E.; Healey, M.; Anderson, R.; Sørhaug, E. Crystal Growth Inhibition Studies for the Qualification of a Kinetic Hydrate Inhibitor under Flowing and Shut-In Conditions. Energy Fuels 2014, 28, 2902−2913. (22) Sheng, Q.; Silveira, K. C. d.; Tian, W.; Fong, C.; Maeda, N.; Gubner, R.; Wood, C. D. Simultaneous hydrate and corrosion inhibition with modified Poly (vinyl caprolactam) polymers. Energy Fuels 2017, 31, 6724−6731. (23) Burgazli, C.; Navarrete, R.; Mead, S. New dual purpose chemistry for gas hydrate and corrosion inhibition. J. Can. Pet. Technol. 2005, 44, 1. (24) Akhfash, M.; Arjmandi, M.; Aman, Z. M.; Boxall, J. A.; May, E. F. Gas Hydrate Thermodynamic Inhibition with MDEA for Reduced MEG Circulation. J. Chem. Eng. Data 2017, 62, 2578. (25) Achour, M.; Kolts, J. Corrosion Control by Inhibition Part I: Corrosion Control by Film Forming Inhibitors. In Corrosion; NACE International: Dallas, TX, 2007. (26) Alef, K.; Smith, C.; Iglauer, S.; Gubner, R.; Barifcani, A. The effect of regenerated MEG on hydrate inhibition performance over multiple regeneration cycles. Fuel 2018, 222, 638−647. (27) Smith, C.; Barifcani, A.; Pack, D. Gas hydrate formation and dissociation numerical modelling with nitrogen and carbon dioxide. J. Nat. Gas Sci. Eng. 2015, 27, 1118−1128. (28) Mech, D.; Pandey, G.; Sangwai, J. S. Effect of molecular weight of polyethylene glycol on the equilibrium dissociation pressures of methane hydrate system. J. Chem. Eng. Data 2015, 60, 1878−1885. (29) Calsep PVTSim, 14.1.0; CALSEP A/S: Houston, TX, 2011. (30) Infochem Multiflash, 3.6.39; Infochem Computer Services Ltd.: London, U.K., 2007. (31) Sloan, E. Clathrate Hydrates of Natural Gases; Marcel Deckker Inc.: New York, 1998. (32) McLeod, H. O., Jr; Campbell, J. M. JPT, J. Pet. Technol. 1961, 13, 590−594. (33) Marshall, D. R.; Saito, S.; Kobayashi, R. Hydrates at high pressures: Part I. Methane-water, argon-water, and nitrogen-water systems. AIChE J. 1964, 10, 202−205. G


Hydrate Phase Equilibria for Methyldiethanolamine and Empirical

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